Field of the Embodiments
The various embodiments relate generally to audio signal processing and, more specifically, to techniques for hybrid active noise control (ANC).
Description of the Related Art
Many techniques have been developed for eliminating unwanted noise in various environments. In one such technique, known as active noise cancellation (ANC), noise in the surrounding environment is detected via one or more microphones. Inverses of one or more waveforms associated with the noise are then generated and reproduced via one or more speakers in order to destructively interfere with or “cancel out” the noise. Such techniques are employed in a wide range of devices, including ANC headphones and hearing aid devices, providing users with greater control over their auditory environments.
Recently, ANC systems have begun to be integrated into larger systems, such as automobiles. In particular, in automotive applications, multiple microphones are distributed throughout the vehicle and passenger compartment, and acoustic data acquired via the microphones are transmitted to a centralized control unit. The centralized control unit then generates noise cancellation signals, which are reproduced by speakers within the passenger compartment of the vehicle to cancel out the vehicle and road noise detected by the microphones.
Although ANC techniques are relatively effective in reducing unwanted noise in relatively smaller environments, such as within automobiles, these techniques are less effective at reducing noise in larger environments and at higher frequencies. In particular, increasing the acoustic volume results in an exponential increase in the modal density within the environment. The corresponding large number of participating acoustic modes typically requires that the number of speakers be at least equal to the number of relevant acoustic modes. For example, and without limitation, the number of acoustic modes as a function of frequency (Hz) for a helicopter cabin is shown in FIG. 6. Because of the increased modal density within the environment, generating noise cancellation signals within that environment via a centralized control unit, as described above, becomes prohibitively complex.
In order to address these issues in large listening environments, some conventional approaches implement a fully decentralized ANC system having multiple control units, where each control unit operates independently of the other control units. In particular, to gain control authority in such decentralized ANC systems, a different control unit typically is required for each participating acoustic mode within the listening environment. In view of this constraint as well as the increased modal densities in larger listening environments, decentralized ANC system implementations in larger listening environments usually have significant hardware requirements, making such systems cost-prohibitive in such environments. Further, the weight associated with decentralized ANC systems makes these systems impractical for use in transportation-oriented environments, such as in aircraft and automobiles.
As the foregoing illustrates, more effective techniques for performing active noise cancellation in various types of listening environments would be useful.